For a magnetic field along $\hat{x}$ the hamiltonian is given by: $H_{z}=V_x(c^{\dagger}_{\uparrow}c_{\downarrow}+c^{\dagger}_{\downarrow}c_{\uparrow})$.
If we follow the usual prescription of changing the spins: $\uparrow\quad \rightarrow \quad\downarrow$ and vice-versa, how do we see that the above hamiltonian breaks TR symmetry?